The differential display technique was applied for identification of genes that have altered expression in mouse hepatocellular carcinomas relative to normal liver. Three genes were identified. The IL-1 receptor antagonist (IL-1ra) was expressed in hepatocellular carcinomas, whereas the major urinary protein (MUP) and cytochrome P-450 naphthalene hydroxylase (cyp2F2) genes were down-regulated. Because IL-1ra is a natural antagonist of IL-1, and because the latter has been reported to suppress the growth of hepatic cells, we also studied the expression of IL-1ra in hepatocarcinogenesis. IL-1ra was immunohistochemically detected in tumor cells in approximately 70% of hepatocellular adenomas and carcinomas, whereas early preneoplastic hepatocytic foci, as well as normal hepatocytes surrounding the lesions, were negative. In addition, 20% of human hepatocellular carcinomas were also partly positive for IL-1ra. RT-PCR analysis demonstrated that mouse hepatic tumors contain both secreted and intracellular forms of IL-1ra. On the other hand, there were no differences in levels of IL-1alpha and IL-1beta between hepatic tumors and normal liver in mice, suggesting that the majority of tumors create a microenvironment that inhibits the actions of IL-1. Furthermore, IL-1ra-positive adenomas contained more proliferating cell nuclear antigen-positive cells than IL-1ra-negative adenomas, indicating a link with high proliferation activity, although this was no longer evident in carcinomas. The observed altered gene expression may be related to biological phenotypes of hepatic tumors, and IL-1ra in particular may positively influence tumor cell growth through its antagonism of IL-1.